Efforts to prevent mental health problems in youths have advanced greatly in recent years. However, these advances have not reduced rates of youth mental illness on a large scale. Thus, a great need exists for novel, scalable, and low-cost approaches to preventing mental health problems in youth. Ideally, such approaches would be mechanism-targeted: that is, they would act on specific developmental processes that underlie psychological disorders. The proposed research aims to address this need by testing whether a single-session intervention teaching incremental theories of personality, or the belief that one's personality is malleable-as opposed to entity theories of personality, or the belief that one's personality is fixed and unchangeable-can strengthen recovery from social stress and prevent the development of anxiety and depression during early adolescence. Compared to incremental theories, entity theories of personal traits have demonstrated cross- sectional and prospective relations with greater anxiety and depression in youths. Further, a single-session incremental personality theories intervention reduced the development of depressive symptoms in a community sample of adolescents, supporting these theories as powerful prevention targets, even when taught in a brief format. Specifically, in line with the NIMH's experimental therapeutics approach to testing novel interventions, this project has two aims. Aim 1 is to evaluate the effect of the implicit theories intervention on two candidate mechanisms of action, or targets, identified by prior research: arousal (measure via physiological reactivity following social stress, as well as self-reported anxiety) and loss (here, perceived loss of behavioral control) in youths 12-15 years of age. Following a lab-based social stress induction, this study will test whether participants receiving the intervention (N=45) will recover from stress more rapidly, as indicated by measures of arousal (heart rate variability; respiratory sinus arrhythmia; galvanic skin response) and self- reported loss (increased perceived control at post-intervention or 3-, 6-, and 9-month follow-ups) compared to participants who do not receive the intervention (n =45). Aim 2 is to evaluate the effects of the single-session incremental theories intervention on anxiety and depression over nine months. This study will test whether the intervention, compared to a control protocol, prevents the development of anxiety and depression; it will also assess whether this change is a direct result of shifts in the two aforementioned targets (arousal; loss). More positive trajectories in anxiety and depression are predicted for youth receiving the intervention, relative to those in the control condition, over time. The study will aso test whether these trajectories are mediated by changes in the targets described in Aim 1. Finally, regardless of outcomes for Aims 1 and 2, baseline, postintervention, and 9-month measures will be used to map longitudinal links among implicit theories, interventions targeting those theories, social stress recovery, and youth anxiety and depression. Findings may suggest a cost-effective, scalable intervention that improves youth resiliency and mental health.